Studies of controllable ambipolar characteristics and mechanisms of pentacene-based organic thin-film transistors

  • 邱 亮雲

Student thesis: Doctoral Thesis


This thesis focuses on the controllable ambipolar characteristics and mechanisms in pentacene-based organic field-effect transistors (OFETs) We explored charge transport (CT) in organic semiconductors (OSCs) as well as key factors on device fabrication to improve ambipolar characteristics in molecular-scale views by optimizing the interface property in interfacial free energy Ambipolar electrical characteristics such as mobility threshold voltage on/off state ratio and subthreshold swings were controlled and adjusted through the dual-carrier recombination and release (DCRR) process The DCRR hypotheses interpret that opposite sign charges can capture and release major charges and thus significantly influence electrical characteristics These hypotheses were verified by the results of different electrical measurements of applied lateral and vertical electrical fields on a device with different bias sweep histories and hysteresis phenomena and the replaced metals as electrodes The dielectric sandwich structures inserted with a ferroelectric polymer layer (FPL) in pentacene-based OFETs were demonstrated for improving the n-channel and the balanced electric characteristics Information on the trap density and charge behavior in an interface prototype was also obtained by the capacitance-related measurement The correlations of film features microstructural quality and crystalline growth in pentacene with ambipolar characteristics were also investigated Part I reports the ambipolar CT modulation of OSCs in field-effect transistors (FETs) based on joint experimental and theoretical studies Pentacene one of the most widely studied OSCs is used as an example in this study to achieve efficient and balanced ambipolar FETs (i e electron mobility is as fast as hole mobility) via the fine control of interfacial properties of the active channel An electron and hole mobility of >2 5 cm2/Vs and a balanced ratio of approximately 1 can be achieved by eliminating the interfacial free energy of the active channel Theoretical investigations of the pentacene molecule show that the electron density is mainly located on the terminal ends but the hole density is located on the three centered aromatic rings This finding provides insights into the difficulty of observing high electron mobility levels compared with hole mobility in FETs An analysis of theoretical vibrational reorganization energy supports that the electron transport between pentacene molecules suffers from more serious interfacial effects than the hole transport With the help of theoretical calculations Raman spectroscopy can be used to characterize pentacene films at an early stage of growth in nanometer scale and assess the ambipolar transport potential in FETs The results demonstrate an excellent correlation between ambipolar characteristics in actual organic FETs and microscopic molecular-level properties In further practical applications an organic complementary-like inverter is integrated with two identical ambipolar transistors The modified inverter shows good static transfer characteristics and high voltage gain over 40 dB Part II presents the determination of the electrical characteristics of pentacene-based ambipolar OFETs through the use of appropriate dielectrics of poly(methyl methacrylate)(PMMA) Results show that the electrical characteristics of pentacene-based ambipolar OFETs significantly differ from those of unipolar OFETs The electrical and hysteresis characteristics of the ambipolar OFETs depend on the applied source-drain bias and sweeping gate voltage history The DCRR hypotheses are thus proposed to illustrate the DCRR process in the transfer characteristics step-by-step The prepared balanced ambipolar OFETs show an electron and hole mobility of >1 5 cm2/Vs as a compared sample to interpret the difference in electrical phenomena compared with the one in unipolar OFETs with SiO2 dielectrics The effects of charge injection and parasitic contact resistance on the ambipolar characteristics are analyzed by trap-charge-limited theory and the transmission line method Replacing different metals (Ag Au and Ca) as electrodes can converse the ambipolar transport of pentacene to unipolar and can be applied into the corresponding OFETs to verify the DCRR hypotheses Part III highlights the functions of FPL in a dielectric sandwich structure to improve the n-channel electrical characteristics and achieve high-performance balanced ambipolar OFETs The sandwich dielectrics reinforce the effective electric fields causing the net electrical field strength to induce extra charge density accumulations and thus an early threshold voltage to the n-channel characteristics Polarization density–electric field analysis reveals that the overall electric susceptibility of the OSC sandwich structure for electron accumulations is rapidly increased as the applied electrical field aligns the dipole within the FPL along the same direction However the linearity is retained for the structure without OSC The dipole effects of FPL are more significantly exhibited on the n-channel operation than on the p-channel operation The novel polarized dielectrics are proven to be good insulating materials for next-generation photonic and micro-electronics because of their good charge-stored capability simple fabrication and power saving capacity
Date of Award2014 Mar 27
Original languageEnglish
SupervisorHorng-Long Cheng (Supervisor)

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